(1) Field of the Invention
The present invention relates to an arc tube to be used for a low-pressure mercury discharge lamp, the arc tube having an arc-tube body whose interior space is for forming one discharge path and electrodes provided at ends of the arc-tube body that correspond to ends of the interior space. The present invention also relates to a low-pressure mercury discharge lamp employing the arc tube.
(2) Related Art
Recently, in this energy saving era, with a view toward saving energy also in the lighting field, more and more compact self-ballasted fluorescent lamps are being developed for widespread use as energy-efficient light sources replacing incandescent lamps.
Compact self-ballasted fluorescent lamps consume less power than incandescent lamps, which is advantageous in energy saving. However, being larger than incandescent lamps in size, compact self-ballasted fluorescent lamps have not become widespread because sometimes they cannot be mounted to an existing illumination device, and other times even if they fit the illumination device, a tip thereof protrudes from the illumination device.
In view of this problem, so as to bring compact self-ballasted fluorescent lamps into widespread use, both of size reduction comparable to that of incandescent lamps and further energy saving for lamp efficiency enhancement have been considered (e.g. Japanese Laid-open Patent Application No. 2003-263972).
FIG. 1 shows main dimensions, lamp efficiency, etc. of compact self-ballasted fluorescent lamps (corresponding to incandescent lamps of 60 W type) respectively released in the years of 1996, 2000, and 2004. Note that an incandescent lamp of 60 W type has a maximum outer diameter (corresponding to “D1” in FIG. 2) of 60 mm and a length (corresponding to “L1” in FIG. 2) of 110 mm.
As shown in FIG. 1, the compact self-ballasted fluorescent lamp released in 1996 has a maximum outer diameter of 60 mm and a length of 135 mm, being substantially larger than an incandescent lamp. The lamp efficiency of this compact self-ballasted fluorescent lamp is 57.9 lm/W.
Compared to this, the compact self-ballasted fluorescent lamp released in 2004 has a maximum outer diameter of 55 mm and a length of 110 mm, attaining size reduction up to the same level as an incandescent lamp. The lamp efficiency of this compact self-ballasted fluorescent lamp is 67.5 lm/W, showing 17% improvement compared to the lamp released in 1996.
The size reduction for these lamps was pursued by narrowing the glass tube constituting an arc tube. Meanwhile, the energy saving for these lamps was pursued by narrowing the arc tube and also by increasing the length of its discharge path (distance between electrodes) thereby reducing the tube wall loading. Here, tube wall loading is obtained by dividing the arc-tube power by an inner surface area of the glass tube portion corresponding to the discharge path.
In this way, the compact self-ballasted fluorescent lamp has achieved size reduction up to the level of an incandescent lamp, as well as high lamp efficiency.
However, in the recent market of light bulbs, it is mini krypton lamps that are becoming mainstream, which are smaller than incandescent lamps having been mainstream in the market. Regarding the dimensions of a mini krypton lamp in the case of 60 W type for example, the maximum outer diameter is 35 mm and the length is 67 mm. Note that the luminous flux of a mini krypton lamp of 60 W type is 810 lm, just the same as the luminous flux of incandescent lamps and compact self-ballasted fluorescent lamps stated above.
In such a circumstance, in an attempt to obtain a small self-ballasted fluorescent lamp having a size comparable to that of a mini krypton lamp, the stated conventional method of narrowing the glass tube and increasing the length of the discharge path has been pursued. However, it has not been successful because the length of the glass tube became too long while narrowing the glass tube.